What are some good arguments against fornication without conflating with hookup culture?

I think that hookup culture is definitely bad for mental health, long term pair bonding, and a number of psychological reasons.

I think Conservatives have done an excellent job refuting hookup culture and explaining why it's really problematic.

However, I think Conservatives sort of have not fully refuted fornication through medium length relationships. For example, if somebody was boyfriend girlfriend for 5 years or so, and fornicated inside of this relationship.

Can you give good conservative refutations of medium length relationship fornication, without conflating data from hookup culture, because that's a disingenuous thing to do.

I'm curious to hear some well written out answers, because I sort of agree with the Conservative view on this, but I don't know if the evidence is that clear yet.